By using the indirect method of fluorescent staining to study the antibody response in patients with primary atypical pneumonai associated with the development of cold agglutinin, it was found that the PAP antibody developed during the 2nd and 3rd week of the illness, and persisted for over a year, and is not related to the cold and streptococcus MG agglutinins. The development of the PAP fluorescent staining antibody paralleled the neutralizing antibody for the PAP virus as tested in cotton rats. The sensitivity of this specific serological test was indicated by the observation that 67 to 92 per cent of the patients in several outbreaks of PAP showed a rise of antibody titer during convalescence. Absorption of the sera with various tissue powders did not affect the PAP antibody detected by this method.

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